IEC defined 60034-1 S1 …S10 Duty Types and their applicability to mechanical devices.

By Dr. Gerhard G. Antony, Neugart USA Corp

The paper presents and explains the IEC defined duty types and their relationship to the rating. It

presents and discusses the fundamental performance / loadability limiting phenomena for electrical and

mechanical devices. Finally it is comparing which from the defined S1 to S10 IEC duty types is applicable

to mechanical devices. As a conclusion it clearly states that only the S1 Duty Type has a transparent

applicable meaning for gearboxes.

IEC S1… S5…S10: Duty Types and their applicability to mechanical devices. Scope For machines, devices, and equipment their rated performance data is the most important description. The rating shall clearly and unambiguously define and reference to the conditions at which the device will function as specified, and warranted by the manufacturer. In practical usage, applications there are countless possible ways to run a machine; changing loads and speeds, different idle times, different peak loads of different duration etc. The possible “Duty Types” for a machine such as a standard AC electric motor are virtually countless. To make selection, rating, and comparison possible and transparent the IEC defined a set of characteristic Duty Types in its International Standard 60034-1. Obviously, for an electric motor the application / duty is described by loads and their change and duration, in most cases it is assumed that this characteristic duty is cyclically repeating in the same or very similar form. See below the IEC defined Duty Types, called S1 to S10. The definition - graphs showing the load as Power P, the Power Losses, Pv and the associated Temperature / Temperature-Rise of the motor over the time.

Continuous running duty

Intermittent periodic duty with electric

braking

S2

Short-time duty

S6

Continuous operation periodic duty

S3

Intermittent periodic duty

S7

Continuous-operation periodic duty with

electric braking

S4

Intermittent periodic duty with

starting

S8

Continuous operation periodic duty with related load/speed changes

S9 Duty with non-periodic load and speed variations

S10 Duty with discrete constant loads

What is limiting the loadability? Electrical machines The fundamental load limiting factor of electrical machines is their thermal loadability. In motors the heat-up is caused basically by the Ohmic-Losses in the winding. Power losses are increasing exponentially with the current / load Pv= RI2 .To avoid a premature break down of the insulation, excessive heat-up must be avoided, i.e. certain operating temperature should not be exceeded. Different operating cycles (Duty Types) cause different heat up. A high peak load

torque with associated high currents can cause a fast and strong temperature increase, i.e. the duration of it must be limited. Subsequently one must allow the motor to cool down before it can be subjected to another such cycle. Mechanical devices The fundamental load limiting factor of mechanical devices is the magnitude of the Mechanical Stresses, created by the forces, bending and twisting moments, and surface pressures in the components. It is dependent upon the material if it can withstand, and for how long, the repeated stressing. A mechanical component, device will not “recover” from a high peak stress cycle like a motor which will regain its full lodability after giving time to cool down. A mechanical component will develop micro-cracks at each load cycle and suffer “fatigue fracture” if the stressing exceeds its endurance limit. Hence the stresses must be limited to avoid premature failure. This behavior is commonly expressed by the S-N curve which is a plot of the load (Stress magnitude),S- over the number of endured cycles, N.

The S-N curve The basic shape of the S-N curve for metals is shown in the above Figure. Whereas Steel an alloys of steel have a very distinct endurance limit (fatigue limit) other metals such as aluminum have less distinct “horizontal” part in their S-N curves. The actual shape of the S-N curves depend from many factors, first of all from the material composition, heat treating condition, type of loading (bending, tension, sheer) etc. Gears and Electric Motors In very many applications electric motors cannot develop sufficient torque at the rpm the application demands, their torque needs to be boosted. Furthermore mostly a sturdy mechanical interface is needed to facilitate a connection to the machinery to perform the mechanical work. Combining the motor and gearbox one has to remember of the fundamental differences which is limiting their loadability!

It would be so very convenient to use the IEC defined Duty Types to publish gearbox ratings based on them. This would be however incorrect, and irresponsible. Because of the completely differ torque limiting phenomena, thermal versus mechanical stress only one of the IEC descried Duty Type is suitable namely the S1 – Continuous Duty. All the other, S2 to S10 duty types can have peak loads causing material fatigue. Strictly, the only way it could be permissible reference to other than S1 duty is, if the load listing as clearly associated with the number of load cycles the mechanical device (gearbox) can endure. For design and dimensioning of gearboxes there are numerous established rating standards from AGMA, ISO, DIN. If the load exceeds the endurance limit the only responsible and permissible method would be determine and publish a rating based on a certain clearly defined load cycle using a suitable “Cumulative Damage Model” such as the Miner’s Rule. Oddly enough some servo gearhead manufacturers are listing ratings referencing to some imaginary not clearly defined S5 duty type. The motivation behind this practice is more than questionable. It means, that during the sizing selection the manufacturer listed inflated ratings are tweaked by not transparent “de-rating / sizing factors”, published by the manufacturer. To make sizing transparent Neugart lists the rated torque referencing S1 type continuous duty, at a defined rpm. Neugart is also lists a peak torque value, higher than the endurance limit of some components which the gearbox will endure at least 30000 output shaft rotations. When selecting and comparing servo-gear-heads, beware how the published ratings are

defined!